1. Field of the Invention
The present invention relates to scatter-glare correction for x-ray desitometry using deconvolution techniques, and in particular to deconvolution techniques using a calibration phantom and a regularization technique.
2. Description of the Prior Art
X-ray imaging is widely used in medical diagnostics and other fields of application. However, an x-ray image obtained from a subject such as a patient is degraded by many factors resulting in artifacts in the image. For example, an x-ray image of a patient is degraded by x-ray scattering within the patient. As a result of this scattering, the image obtained using an x-ray detector is a superposition of a primary component, or the image of the subject, and a scattered component associated with the primary component. In x-ray fluoroscopy where an image intensifier is used in front of the x-ray detector, images are further degraded by veiling glare in the image intensifier. Veiling glare arises from the scatter of electrons and photons within the x-ray image intensifier and causes the electrons and photons produced from the brighter areas of the image to be spread into the darker areas, resulting in a blurring of the image.
These artifacts must be corrected before accurate quantitative information can be derived from the images. Various post-processing techniques have been developed to remove the scatter and glare effects from recorded x-ray images, such as deconvolution. Deconvolution techniques employ a convolution model to describe the scatter-glare component. Mathematically, the scatter-glare component of an image can be modeled by a spatial convolution of the primary component with a two-dimensional response function. When the response function is known, an estimate of the primary component of the image can be calculated from the measured image intensities by a deconvolution technique (also known as Wiener filtering or constrained restoration). Scatter-glare correction using deconvolution techniques is described in, for example, J. A. Seibert et al., Removal of Image Intensifier Veiling Glare by Mathematical Deconvolution Techniques, MED. PHYS., 12(3), 281-288 (1985); S. Naimuddin et al., Scatter-Glare Correction Using a Convolution Algorithm with Variable Weighting, MED. PHYS., 14(3), 330-334 (1987); L. A. Love et al., Scatter Estimation for a Digital Radiographic System Using Convolution Filtering, MED. PHYS., vol. 14(2), 178-185 (1987); L. A. Seibert et al., X-ray Scatter Removal by Deconvolution, MED. PHYS., 15(4), 567-575 (1988); and Scatter-Glare Corrections in Quantitative Dual-Energy Fluoroscopy, MED. PHYS., 15(3), 289-297 (1988). Various functions may be used for the response function, such as exponential or Gaussian functions. The parameters of a response function are usually determined empirically. Ideally, because the response function for each image is dependent upon various acquisition parameters such as beam energy, imaging geometry and patient thickness, which are variable from patient to patient, the precise response function parameters should be determined for each individual patient to achieve accurate scatter-glare removal.
Various methods for determining the response function parameters have been developed. In the methods described in the above references, the response function parameters are estimated by using an object such as a Lucite slab to mimic a patient, placing a series of lead objects before the Lucite object to completely block the direct x-rays in parts of the image, and measuring the x-ray intensities in the blocked areas of the image which presumably consist solely of the scatter-glare component. These methods only give the estimated parameters for an average or typical patient.
To account for dependence of the response function parameters on acquisition parameters, one deconvolution method allows variation of the response function parameters based on empirical relationships between the acquisition parameters and the response function parameters. However, this method still does not allow response function parameters to be optimized for each individual patient. In another method, an array of beam stops is placed between the x-ray source and the patient when the patient is being imaged to block the direct x-ray, and the response function parameters for the particular patient are determined by comparing the image intensities behind the beam stops (which presumably would be scatter-glare only) with scatter-glare intensities calculated from the convolution model. A disadvantage of this method is a loss of information due to complete blocking of parts of the image. Other techniques involve taking multiple images of the patient, such as with and without beam stops, to estimate the parameters. These methods suffer the disadvantage of increased patient radiation dose and data acquisition time.
Thus, each of these methods suffers one or more of the following disadvantages: (1) the response function parameters are not optimized for each individual patient being imaged, thus reducing the accuracy of the scatter-glare removal; (2) more than one image must be taken to obtain one corrected image, thus increasing patient radiation dose and the length of the x-ray process, and reducing throughput in the x-ray process; (3) the image of the patient is completely blocked in some areas by beam stops or other lead objects placed in the beam. These and other disadvantages often make these techniques unsuitable for clinical applications.
Therefore, it is an object of the present invention to provide a method and apparatus for scatter-glare correction for x-ray images suitable for clinical applications. It is another object of the present invention to provide a method and apparatus for scatter-glare correction without requirements of specific prior knowledge about the subject. It is a further objective of the present invention to provide a post-processing method for scatter-glare correction for a single x-ray image without the need for multiple images. It is yet a further objective of the present invention to provide a method and apparatus for scatter-glare correction without using beam stops or other objects that block parts of the subject image.